1. Field of the Invention
The present invention relates to the employment of multipolymers comprising vinylidene chloride based polymers as coatings for thermoplastic packaging films. Such coatings contain incorporated therein minor amounts of a novel anti-static additive material employed to reduce the tendency of such film materials to accumulate electrostatic charges and thus attract and hold particulate materials. The anti-static characteristics of the coating additionally reduce the film's undesirable tendency to cling or adhere to the surfaces of automatic packaging equipment.
2. Brief Description of the Prior Art
The employment of multipolymers of vinylidene chloride, hereinafter referred to as saran, as coatings for oriented polypropylene materials has been recognized in the prior art as an effective means for increasing the resistance of such film materials to the transmission of gases and vapors, such as oxygen and moisture vapor for example. Additionally, such saran topcoating materials have been known to promote the heat sealability of such oriented film structures which, in an uncoated state, tend to seal only with great difficulty, if at all. Saran coated oriented polypropylene is a particularly good packaging material for products which tend to be sensitive to attack by oxygen such as, for example, coffee and cheese. However, such saran coated plastic materials have a tendency to develop a static charge when used on packaging equipment. This has been an obstacle particularly when packaging ground coffee and other particulate products in that the static charge developed on the film tends to attract the product to the seal area thus preventing the packaging film from effectively sealing to itself in a continuous manner.
In the past such coated packaging film has been produced by subjecting the surface of an oriented film to a corona discharge treatment to pretreat the surface in order to improve its receptivity, i.e., bonding characteristics, to coatings which may be subsequently applied thereto. Other surface pretreatment techniques which may be employed include flame treatment or treatment of the film surface with oxidizing chemicals such as chromic acid for example. The thus treated oriented polypropylene may then be coated with a thin layer of a primer material which serves to enhance the adherence of the saran topcoat layer, subsequently supplied to the base film. Primers which have been used in the past include polyurethane dissolved in organic solvents, see for example U.S. Pat. No. 3,023,125. Alternatively, the saran coating may, in certain instances, be applied directly on the treated film surface without the employment of a special primer coating. A wide variety of saran topcoat materials are currently commercially available. Any one of these sarans may be employed in the practice of the present invention and preferably those which have a vinylidene chloride content of about 75% by weight up to about 92% by weight. Any one of a number of well known coating additives may also be incorporated in the saran topcoat material. Adjuvants such as carnauba wax, employed to promote cold slip and reduce blocking tendencies; additionally materials such as talc employed to improve the frictional characteristics of the film surface, and many other prior art materials may be added to the particular saran latex employed to achieve the desired coating characteristics.
British Pat. No. 974,116 teaches the use of cationic surface active agents as antistatic additives for polymeric coatings for polyolefin films, specifically quaternary ammonium compounds. However, these materials are well known to exhibit high levels of toxicity and their use in packaging films for food products is not acceptable.
Conventional anti-static agents for plastic films such as glycerylmonostearate and high molecular weight secondary or tertiary amines, e.g., U.S. Pat. No. 3,663,488, have been found to be impractical for use in coated films. In the first place, they are difficult to disperse uniformly in aqueous latex coatings and secondarily, they destroy the essential heat seal properties of the resultant films. In contrast the coated films prepared in accordance with the present invention exhibit excellent heat seal properties.
Water soluble non-ionic surface active agents such as nonylphenoxy poly(ethylene oxide) have been claimed to be effective anti-stats when admixed with a saran topcoat on a modified polypropylene base sheet but these destroy heat sealability when applied to a 100% polypropylene base sheet. Note Japanese Pat. Nos. 49-38021 and 52-50232.